1. Field of the Invention
The present invention relates to a lamp used as light source for the device to measure atomic absorption of light, and more particularly to a light source lamp used in an atomic light absorption measurement device utilizing Zeeman effect.
2. Description of the Prior Art
The atomic light absorption measurement device is used mainly for the analysis of metallic elements. A metal compound can be decomposed into the vapor of atoms of constituent metallic elements at their ground state, by placing the compound in flame or a furnace. If the light emitted from the atoms of the same metal as one of the metallic elements are passed through the vapor of atoms at their ground state, the atoms of the one metallic element absorb a part of the light, with the remaining part of the light transmitted. The quantity of light absorbed by the atoms of the elements to be analyzed can be known from the variation of the intensity of the transmitted light with respect to the intensity of light emanating directly from the source. Since there is a proportional relation between the amount of light absorbed and the concentration of the atoms of the element to be analyzed, the quantity of the element to be analyzed can be determined from the amount of the light absorbed. The atomic light absorption measurement device is on the basis of the principle described above.
In an atomic light absorption analyzer, the apparent absorption of light in a sample due to dispersion and the molecular absorption of light contribute to an error from the true amount of light absorbed by the element to be analyzed. The error is appreciable enough so that it cannot be usually neglected. In general, the width of the absorption spectrum of an element to be analyzed is very narrow while the apparent absorption due to light dispersion or the molecular absorption takes place over a considerably wide range of wavelengths. So, in order to eliminate the aforementioned error, the light having a wavelength coincident with that absorbed by the element to be analyzed is used as sample light and the light having a wavelength different from that absorbed by the element to be analyzed is used as reference light, so that so-called two wavelength comparison measurement due to the comparison of the sample light to the reference light is performed. This comparison can be done in practice by making the ratio of or the difference between the intensities of the reference light and the sample light having passed through the sample.
It is preferable that the wavelength of the sample light is as near to that of the reference light as possible so far as the former is not coincident with the latter. Namely, it is preferable that since the width of the absorption spectrum of the element to be analyzed is, in terms of wave number usually less than 1 cm.sup.-1, a wave number difference as small as 1 cm.sup.-1 should exist between the sample light and the reference light. This is due to the fact as follows. Although such backgrounds as the apparent and the molecular absorptions occur over a comparatively wide range of wavelengths, they are not necessarily equal in amount for different wavelengths. Therefore, the effect of eliminating or compensating the backgrounds is greater when the wavelengths of the sample and the reference lights are as near to each other as possible than otherwise.
If a light source, which emits a single spectral line in the absence of magnetic field, is placed in magnetic field, a plurality of spectral lines are absorbed. Namely, one is the spectral line identical with the original spectral line observed in the absence of magnetic field and the other are the two components appearing a little separate in wavelength from and in symmetry with the original spectral line. The spectral line coincident with the original one is termed a .pi.-component (corresponding to a magnetic quantum number variation .DELTA.m = 0) and the two symmetric spectral lines are referred to as .sigma..+-. and .sigma..sup.- -components (corresponding respectively to .DELTA.m = .+-.1). It is known that the intensity of the .pi.-component is theoretically equal to the sum of the intensities of the .sigma..+-. and .sigma..sup.- -components. This phenomenon is known as Zeeman effect and only the .sigma..+-. and .sigma..sup.- -components, which are respectively right-handed and left-handed, circularly polarized lights, are observed in the direction parallel to the magnetic field while in the direction perpendicular to the field the .sigma..+-. and .sigma..sup.- -components, which are linearly polarized lights having a plane of oscillation perpendicular to the field, and the .pi.-component, which is linearly polarized light having a plane of oscillation parallel to the field, are all observed. The wavelength of the .pi.-component remains unaltered, independent of the degree of Zeeman effect but the deviation in wavelength of the .sigma..+-. and .sigma..sup.- -components from the .pi.-component depends upon the intensity of the magnetic field. Namely, it is known that there is a relation .DELTA..nu..varies. H, where .DELTA..nu.(cm.sup.-1) is the deviation in terms of the change in wavenumber and H is the intensity of the magnetic field. Accordingly, if the .pi.-component is used as sample light and the .sigma..+-. and .sigma..sup.- -components are both used as reference light or if the .sigma..sup.- -component is used as sample light and the .sigma..sup.+ -component as reference one, then the aforementioned two waveform comparison measurement is possible. It is, of course, easy to make the difference in wave number between the sample light and the reference light less than 1 cm.sup.-1. Thus, the above description shows how useful the two wavelength comparison measurement utilizing Zeeman effect is.
In a device for measuring the atomic absorption of light, a lamp is used as light source, which comprises a bulb filled with such inert gas as argon and a cathode and an anode arranged in the bulb. In such a lamp, the cathode is sputtered by the positive ions produced as a result of the ionization of the inert gas due to the discharge between the electrodes and the atoms of the small particles produced near the cathode due to the sputtering of the cathode are excited by the positive ions produced due to the ionization of the inert gas, the electrons produced due to the ionization of the inert gas and accelerated toward the anode, and the secondary electrons produced as a result of the collision of the positive ions against the cathode and accelerated toward the anode.
Such a lamp has a diameter of at least about 16 mm. A magnet to establish a magnetic field of about 20 K gauss is needed for Zeeman effect to take place appropriately. However, in case where the lamp diameter is about 16 mm, a very large magnet is used to obtain a magnetic field of about 20 K gauss. In order to solve this problem, the size of the lamp may be reduced, but in such a case the decrease in the gas contained in the bulb must be accompanied so that the life of the lamp is shortened.
On the other hand, the inventors have proved through repeated experiments that the intensity of light emitted from the lamp and the luminous stability of the light depend largely upon the direction of the magnetic field and the direction of the electric field established between the cathode and the anode. Namely, in case where the direction of the magnetic field is not parallel to the direction of the electric field, the stability is very poor and the intensity is extremely low or sometimes reduced to zero for an intensity of the magnetic field higher than a certain level, so that the discharge condition attainable when there is no magnetic field cannot any longer be maintained. This is, according to the inventors' investigation, considered to be because the electrons are affected by so-called Lorentz force so that the number of collision of the particles against the cathode per unit time is reduced.